How do I flip a video in the VSE?
In the properties panel N, of the Video Sequence Editor there are some setting for the active strip. Scroll down to the Filter heading and there are two check boxes to flip the strip on the X and the Y axes.




Your other option is to add a transform strip, then use that to rotate your clip.

With the clip you want to rotate selected add a new transform strip by either going to the Video Sequence Editor header Add > Effect Strip > Transform, or by pressing ShiftA.

Then in the same properties panel as in the first part, you will find the settings for the transform strip.